1. Field of Invention
The present invention relates to electro-optical devices using thin-film transistors and methods of manufacturing semiconductor elements, for example.
2. Description of Related Art
Liquid crystal devices are each formed of two substrates, such as a glass substrate or a quartz substrate, and liquid crystal enclosed therebetween. In such a liquid crystal device, active elements, such as a thin-film transistor (hereinafter “TFT”), are disposed in a matrix on one of the substrates; a counter electrode is disposed on the other substrate; and optical properties of the liquid crystal layer enclosed between the two substrates are changed in accordance with image signals, whereby images are displayed.
Image signals are supplied by TFT elements to pixel electrodes composed of ITO (indium tin oxide) and arranged in a matrix, and voltages are applied to the liquid crystal layer between the pixel electrodes and the counter electrode in accordance with the image signals, thereby changing the arrangement of liquid crystal molecules. Accordingly, the transmittance of each of the pixels is changed, and light passing through each of the pixel electrodes and the liquid crystal layer is modulated in accordance with an image signal, thereby displaying an image.
A TFT is placed in an ON state by supplying a scanning signal to the gate electrode through a scanning line, and by supplying an image signal to the source region of a semiconductor layer through a data line, the image signal is supplied to the pixel electrode through the TFT placed in an ON state. Since the supply of the image signal is performed for each pixel electrode within only a very short period of time, in order to maintain the voltage of the image signal for a time much longer than that in which the voltage is applied so that the TFT is placed in an ON state, a storage capacitor is generally added to each pixel electrode.
In the liquid crystal device, between a conductive film, such as an ITO film, forming a pixel electrode and a semiconductor layer forming a pixel switching TFT, a plurality of films is formed having a laminate structure which includes various conductive films, such as a scanning line and a data line, and insulating films, such as a gate insulating film and interlayer insulating films, to insulate the conductive films described above from each other. The distance between the pixel electrode and the semiconductor layer is, for example, approximately 1,000 nm. Hence, from a technical point of view, it has been difficult to electrically connect the pixel electrode and the semiconductor layer to each other with one contact hole. Accordingly, a technique has been developed in which an intermediate conductive layer is formed between interlayer insulating films to electrically connect the pixel electrode and the semiconductor layer.
In the liquid crystal device, higher quality of display image is generally advantageous, and hence, in addition to an increase in pixel aperture ratio (that is, an aperture area through which display light passes is increased in each pixel), it is important that wiring resistances of the various wires, such as the data line, scanning line, and capacitor line, be decreased.
Concomitant with providing a higher aperture ratio of pixels which are provided at fine intervals, the line widths of the data line and the scanning line itself can also be decreased. However, the scanning lines and the data lines are generally formed of a conductive polycrystalline silicon film for the following reasons. For example, (i) heat treatment at a high temperature must be performed after the formation of the scanning lines and the data lines; and (ii) the scanning lines are also used as the gate electrodes of the thin-film transistors.
Accordingly, concomitant with providing a higher aperture ratio of pixels which are provided at fine intervals, as described above, when the line widths of the scanning line and the data line are decreased, or driving frequency is increased concomitant with providing a finer and more accurate structure, the time constant of the capacitor line becomes a problem. That is, when the aperture ratio of the pixel is increased, due to the wiring resistance of the capacitor line, problems become serious in that crosstalk or a ghost is generated in the lateral direction along the scanning lines and in that the quality of display image, such as the contrast ratio, is degraded.
In addition, in a TFT active matrix drive liquid crystal device, when a channel region of a pixel switching TFT provided in each pixel is irradiated with incident light, due to excitation caused by the light, a light leak current is generated, and as a result, the properties of the TFT are varied. In particular, when the liquid crystal device is used for a liquid crystal light valve of a projector, since the intensity of incident light is high, it becomes important that incident light be shaded in the channel region of the TFT and the vicinity thereof. Hence, the structure is formed in that the channel regions and the vicinities thereof are shaded by a shading film which defines the aperture area of each pixel and which is provided for a counter substrate.
In addition, in order to obtain a high aperture ratio of the pixel, that is, in order to increase the ratio of the aperture area of each pixel, a technique has been developed in which, in addition to the shading film provided for the counter substrate, the channel regions and the vicinities thereof are shaded by a shading film embedded in the TFT array substrate or by the data lines which are composed of a metal film, such as Al (aluminum), extending over the TFT.
However, since the distance between the TFT element and the counter substrate or the data line is relatively large, sufficient shading properties have not been obtained by the data lines and the shading film formed on the counter substrate.
In order to address or solve the above, in Japanese Unexamined Patent Application Publication No. 2001-265253, an electro-optical device is disclosed in which, in addition to improvement in shading properties for the TFT elements, a satisfactory storage capacitor can be formed and in which the resistance of the capacitor line can also be decreased.
In this proposal, an intermediate conductive layer having shading properties is formed to electrically connect the semiconductor layer forming the TFT and the pixel electrode. In addition, the storage capacitor is formed by using an upper capacitor electrode and a lower capacitor electrode opposing thereto with a thin insulating film provided therebetween, and a capacitor line formed of the same layer as that for the intermediate conductive layer is connected to the upper capacitor electrode. Accordingly, by using shading areas of individual pixels through which the scanning lines are formed, capacitor lines each having a large width and a large length from the image display area to the periphery thereof can be formed above the scanning lines, and hence, in addition to a sufficient capacitance, a low resistance can be obtained.